Titrations are done to determine the strength of a solution or the concentration of an unknown substance in a solution. Therefore, titrations are analytical procedures that are based on many different chemical reactions that are of interest to the analytical chemist.
A titration may be performed if the chemistry of the desired reaction meets three criteria: the reaction is fast, the reaction proceeds stoichiometrically, and the change in free energy is reasonably large. If these criteria are satisfied then it is possible to determine the endpoint of a reaction by color change of an indicator, by potentiometric means via electrode, by conductivity measurement, by thermal measurement (quantity of heat evolved), and by other means as well.
The typical titration procedure involves the measured addition of a reactant of known concentration (titrant) into a solution that contains a reactant of unknown concentration (sample) until the reaction fully goes to completion (reaches endpoint). A reaction is said to be complete when one of the reactants (sample) has been fully consumed by the reaction process. Since the total volume of the titrant added to the sample at endpoint may be measured and the stoichiometry of the reaction mechanism is known then is a simple matter to determine the exact quantity of reactant within the titrated sample (i.e. unknown concentration of reactant). The most difficult part of this procedure is determining when the endpoint of the titration actually occurs so that the volume may be measured. In the past this has been done manually introducing human error.
For example, a chemist needs to determine the total acidity of a public drinking water supply. She could perform an acid-base neutralization titration in which she would add a titrant (sodium hydroxide of a known concentration) to known volume of the drinking water sample until the total acidity has been determined. Total acidity in natural waters is defined as acidity that comes from mineral acids, weak organic acids and mostly carbon dioxide, (as carbonic acid). Since the phenolphthalein indicator endpoint pH of 8.3 corresponds to the neutralization of carbonic acid to carbonate, the endpoint is an excellent indication of the total acidity of the water sample. By taking a measurement of the total volume of titrant added to reach endpoint, (when the color of the solution changes from colorless to pink) the chemist is able to determine the exact molar strength of the drinking water sample.
There are titration chemistries that rely on color indicators for endpoint determination. Accurate endpoint determination depends upon a suitable indicator that reflects chemical changes that are occurring in the entire volume of the titrated sample. The current generation of automated titrators are severely limited because these instruments have only the capability of detecting color changes in a small fraction of the total sample volume. Consequently, many titration procedures still require a skilled analyst to interpret the point when the indicator's color change signals the endpoint of the reaction. This introduces much human error into the titration. In fact, there are titration methods that depend on color endpoints (such as dissolved oxygen determination by the azide modification of the Winkler Method) that have not been adapted to titration instrumentation.
Chemists have developed hundreds of different indicators which can be used to determine color endpoints of for all kinds of titration chemistries. For example, to perform water and waste water analysis, acidity can be determined by methyl orange, bromphenol blue and total acidity by phenolphthalein, alkalinity can be determined by a bromcresol green-methyl red mixed indicator, and by phenolphthalein or bromphenol blue, carbon dioxide concentration can be determined with phenolphthalein indicator, iodometric titration methods that use starch as an indicator include: free chlorine, chromate, dissolved oxygen, sulfite and others and chloride concentration can be determined by using diphenylcarbazone as an indicator. There are many other endpoint indicators that range over the entire visible spectrum, from red to blue, that are used to determine the concentration of a constituent or the strength of a solution.
At present there exists a need for a reliable, non-invasive means that will detect the color endpoint for the entire sample volume of a titration automatically and with a high degree of accuracy so that laymen and technicians can perform highly accurate titrations without the expertise of a skilled chemist.
There also exists a need for a titration system which can be altered and customized by the highly skilled chemist or research scientist so that specialized titrations may also be automated.